The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
Chemical baths are solutions, normally but not necessarily aqueous, which contain one or more chemicals, hereinafter called chemical species or species. These chemical species exist in certain concentrations in the solution. Some of these species interact or chemically react with a material, called a workpiece, which is placed in the bath. Examples of such chemical baths or “wet processes” are electroplating, electroless deposition, chemical and electrochemical etching, pickling, adhesion promotion, seeding (deposition of a thin catalytic layer on a workpiece), photographic and lithographic developing, and rinsing. Most of these baths interact with the workpiece to produce changes in the workpiece surface, e.g., addition of a film on the workpiece surface or removal of some or all of the initial workpiece surface.
Plating baths, and especially copper plating baths, are described in co-pending application Ser. No. 09/881,817 (allowed Feb. 24, 2003, patent number not yet assigned), the contents of which are hereby incorporated by reference and for all purposes.
While the chemical bath and the workpiece are interacting, both the bath and the workpiece change. Chemical species may leave the bath, i.e., the species is depleted from the bath, to deposit or plate on to the workpiece. Alternatively, a chemical species may be removed (e.g. etched) from the workpiece and enter, or accumulate in the bath. As the bath changes due to the accumulation and/or depletion of these and other species, the bath is said to “age.”
Copper interconnects are widely used for integrated circuits and semiconductor chip structures. The on-chip interconnect lines and vias are formed by single or dual Damascene copper electroplating from an electroplating, or electroless plating, bath. Modern devices are being designed with finer features having larger aspect ratios. Plating such structures using traditional plating baths often results in the formation of voids. Complete fill of the line and via structures is crucial for good electrical performance of the devices. Additives, are compounds added to the plating solution to give the bath so called “superfilling” properties which enable void-free plating of these small features. Two or three organic additive components are commonly used in copper plating chemistries for the microelectronics industry, an accelerator (e.g. sulfide type SPS, (bis-sulfopropyl disulfide)), a suppressor (e.g. PEG type polymer) and a leveler (added when a flat surface is required as an end-product).
Bath metrology techniques such as cyclic voltammetry stripping (CVS) or potentiometric techniques measure and control the concentration of the additive components in electroplating baths. Within a proper concentration range, the combination of these additives results in superfilling of moderate to high aspect ratio vias and trenches. “Superfilling,” also known as “superconformal” or “bottom-up filling,” refers to a process in which the plating rate in a feature, such as a trench or a via, is greater that the plating rate in the plane of the field of the wafer surface. The required differential plating kinetics are obtained by appropriate addition, to the bath, of additives that either inhibit plating in the field or catalyze plating in a feature. During the copper plating process and also during idle periods in between plating, these additives are known to be consumed electrochemically and chemically at the cathode as well as at the anode, bringing reaction products, or so-called byproducts into the solution. While the leached additives are replenished with the help of the bath metrology techniques described above, the byproducts are building up in the solution. This process is known as bath aging. Some of the byproducts will interfere with the superfilling mechanism and as a result may cause voids in the plated copper. Bath exchange or “bleed and feed” is commonly used to keep the byproducts at relatively low concentrations and to obtain steady state conditions. However, the novel copper plating chemistries developed for superfilling of sub micron features are accelerator/suppressor dominated systems as opposed to leveler dominated baths. The rapid gap fill kinetics characteristic for these bath chemistries are capable of filling high aspect ratio submicron features. However gap fill kinetics are very sensitive to accelerator byproducts in solution. Typically the gap fill deteriorates when the byproducts build up in solution. At a certain byproduct concentration the bath will no longer be able to fill small features successfully and plating voids are obtained. This breakdown point depends on the aggressiveness of the trenches and vias (feature size, aspect ratio and shape) and on the concentration of the accelerator. The sulfide type accelerator is the most easily consumed component, both under working as well as under idle conditions. In order to maintain a stable, functional bath it is important to have a technique which can measure the byproducts quantitatively or relative to the accelerator concentration. In this way the bath can be controlled so that by-product levels are kept below the breakdown point or bath threshold for voiding.
The invention comprises a universal method to quantitatively monitor the chemistry of plating baths for by-product levels during bath aging and to eliminate voids in the plated deposit by keeping the by-product levels within a well determined acceptable range. Most commercial available copper plating chemistries (e.g. Enthone-OMI and Shipley Company) contain SPS or an SPS-type sulfide as an accelerator (brightener) in their organic additive package. During the copper plating process and also during idle periods in-between plating the SPS accelerator is consumed and reaction products or so-called by-products are formed. The depleted additives are replenished with the help of the conventional bath metrology techniques. However, the concentration of by-products in solution continues to increase. This process is known as bath aging. High by-product concentrations, particularly in relation to the accelerator concentration, may result into plating voids. Bath exchange or “bleed and feed” is commonly used to keep the by-products at a low steady state concentration.
The copper plating chemistries developed for superfilling of submicron patterns (e.g. Viaform from Enthone OMI and Ultrafill or Nanoplate from Shipley Company) operate with relatively low accelerator concentrations compared to their predecessor formulations. Therefore, as the plating bath ages, the relative concentration of by-products with time may become large as compared to the accelerator concentration. We have observed that increasing ratios of by-product to accelerator concentrations are associated with an increased formation of plating voids.
A method is required to monitor the ratio of by-product concentration to accelerator concentration and thereby to permit appropriate intervention to decrease or prevent void formation.
Other objects and advantages will become apparent from the following disclosure.